Ascent Guidance - MuMech/MechJeb2 GitHub Wiki

The Ascent Guidance of Mechjeb allows to easily get a rocket into a stable, circular orbit with a variety of options. Launches can also be timed to rendezvous with a specific target or can simply be launched when ready.

Table of contents

Launch process

There are four main parts to the launch process, and the exact altitudes at which they occur are customizable:

  1. Initial ascent
  2. Gravity turn
  3. Coast to the edge of the atmosphere (if applicable)
  4. Circularization burn at apoapsis

Main Actions

  • Show navball ascent path guidance
    Creates a targeting reticule on the navball display to point towards while launching (NOTE: this creates a virtual target which will unset any target you currently have set)

  • Engage/Disengage autopilot
    Engages the autopilot or (if engaged) disengages the autopilot

Main Options

  • Orbit altitude [km]
    The desired altitude in kilometres for the final circular orbit

  • Orbit inclination [°]
    The desired inclination in degrees for the final circular orbit

  • Prevent overheats
    Limits the throttle to prevent parts from overheating

  • Limit Q to [pa]
    Limits the maximal dynamic pressure. This avoids that pieces break off during launch because of atmospheric pressure. TODO: specify when to use it

  • Limit acceleration to [m/s²]
    Never exceed the acceleration during ascent

  • Limit throttle to [%]
    Never exceed the percentage of the throttle during ascent

  • Keep limited throttle over [%]
    Never go below the percentage of the throttle during ascent

  • Electric limit Lo [%] Hi [%]
    TODO

  • Force Roll climb [°] turn [°]
    TODO

  • Limit AoA to [°] (0.0°)
    TODO

    • Dynamic Pressure Fadeout [pa]
      TODO

  • Corrective steering
    Will cause the craft to steer based on the more accurate velocity vector rather than positional vector (large craft may actually perform better with this box unchecked)

  • Auto-stage
    The autopilot will automatically stage when the current stage has run out of fuel

    • Delays: pre: [s] post: [s]
      The autopilot will pause the actual staging before (pre) and after (post) for each stage.

    • Clamp AutoStage Thrust [%]
      TODO

    • Stage fairing when:
      Stage the fairing independently of the actual staging process, whenever all (or any? TODO) the following conditions are met:

      • dynamic pressure smaller than [kPa]
      • altitude greater than [km]
      • aerothermal flux smaller than [W/m²]

    • Stop at stage
      Staging will not occur beyond this stage number

  • Auto-deploy solar panels
    Automatically deploy solar panels when safe (verify! TODO).

  • Auto-warp
    Automatically use warp during ascent

  • Skip Circularization
    Do not circularize when apoapsis has been reached

Ascent Profile

There are different ascent profiles available:

  1. Classic Ascent Profile
    TODO Classic Ascent Profile should be more or less what you're used to.

  2. Stock-style GravityTurn™
    This profile is similar to the gravity turn mod. It is a 3-burn to orbit style of launch that can get to orbit with about 2800 dV on stock Kerbin. If you want to have fun make a rocket that is basically a nose cone, a jumbo-64 a mainsail and some fairly big fins. Have the pitch program flip it over aggressively (uncheck the AoA limiter, set the values to like 0.5 / 50 / 40 / 45 / 1) and let it rip. Note that its not precisely the GT mod algorithm, and I didn't intend it to be -- I just didn't know what else to call it. It does not do any pitch-up during the intermediate burn right now (that's another TODO) so it won't handle low TWR upper stages.

  3. Powered Explicit Guidance (RSS/RO)
    PEG is actual gravity turn algorithms from the Surveyor missions that properly integrates the trajectory. It will likely not be that useful on Kerbin since it does not know how to do two-burn to orbit (does not understand coast phases) so most Kerbin launch vehicles won't ever get locked guidance with it. So if you give a 100x100 orbit, then you need a rocket that will burn continuously, without throttling, until insertion where it will shut down right when it hits the 100x100 orbit. Example of it flying a 3-stage rocket to orbit: https://vimeo.com/224742550

Edit Ascent Path

Depending on the profile selection, the Edit Ascent Path button will open a new window with options on the respective profile.

Classic Ascent Profile Options

The options show the diagram of the rocket's idealized ascent. The following options can be edited, and the diagram will update as their values change:

  • Turn Start Altitude
    The altitude at which the gravity turn should begin

  • Turn End Altitude
    The altitude at which the gravity turn should end

  • Final Flight Path Angle
    The final angle with respect to the prograde direction

  • Turn Shape
    Varies the parabolic part of the trajectory

Stock-style GravityTurn

  • Turn start altitude Altitude in km to pitch over and initiate the Gravity Turn (higher values for lower-TWR rockets).

  • Turn start velocity Velocity in m/s which triggers pitch over and initiates the Gravity Turn (higher values for lower-TWR rockets).

  • Turn start pitch Pitch that the pitch program immediately applies.

  • Intermediate altitude Intermediate apoapsis altitude to coast to and then raise the apoapsis up to the eventual final target. May be set to equal the final target in order to skip the intermediate phase.

  • Hold AP Time At the intermediate altitude with this much time-to-apoapsis left the engine will start burning prograde to lift the apoapsis. The engine will throttle down in order to burn closer to the apoapsis. This is very similar to the lead-time of a maneuver node in concept, but with throttling down in the case where the player has initiated the burn too early (the corollary is that if you see lots of throttling down at the start, you likely need less HoldAP time).

Initial Pitch Over Issues

The initial pitch over can cause rockets to tumble. If the rocket gains too much speed and then attempts too large of a pitch maneuver the air resistance will toss it end over end. The solutions to this can be to initiate the turn sooner, use less of a pitch angle, put fins on your rocket to stabilize it, or turn on or tune the AoA limiter.

Gravity Turn Heating / Aerodynamics

After the pitch over the vessel will follow that pitch until the pitch of the surface velocity "catches up with it" (the surface velocity of the navball will "fall" down towards the horizon until it meets the actual pitch of the craft). From there the vessel will follow a zero Angle-of-Attack and will gradually pitch over more and more. If the vessel has insufficient thrust or the pitch program is too aggressive it is possible that the pitch of the craft eventually is level with the horizon and the craft never makes the intermediate apoapsis. It is also possible that the vessel gains too much speed too low in the atmosphere and burns up (flames are actually good, but explosions are not). The solution to this is higher TWR craft or doing a less aggressive pitch over (turning on the AoA limiter also has the effect of limiting how aggressive the pitch over is).

Intermediate Altitude Heating

You can actually burn up at your intermediate altitude if you go too fast, and your speed at that altitude is determined by your target Apoapsis. The default altitude of 45km is good for a 100km-ish target orbit. With a direct launch to a e.g. 10,000km orbit you may need to raise the intermediate altitude.

Time to Orbit Being Long

The default behavior takes a long time because it launches to an apoapsis more than half way around Kerbin from the launch site. This can be avoided by setting the intermediate altitude to the desired altitude and avoiding all the coasting around. It will be slightly less efficient in terms of dV and Oeberth, but substantially faster.

Low TWR Upper Stages and Intermediate Altitude Pitch Up

The MechJeb implementation presently does no pitching up of upper stages at the intermediate altitude. Just a TODO for the future, but really low TWR upper stages may pass through the apoapsis at the intermediate altitude and burn up without pitching up. A higher HoldAP time may fix this.

Launch Training

There is no facility for training the parameters on successive launches like with the GravityTurn mod.

Optimizing for Delta-V

The ideal way to get to orbit with the least amount of dV expended is to:

  • Build a larger rocket (2.5m or larger) with a higher ballistic coefficient (bigger rockets have more mass per surface area).
  • Build a rocket with an excessively high TWR (>4.00 or so if you really want good results)
  • Turn the AoA limiter off
  • Pitch over very aggressively to start with, almost immediately (45 degrees or more)

Lowered intermediate altitudes and lower Hold AP times should also help, but should be fairly secondary in their effect. The point is to minimize gravity losses and trade them for drag losses as much as possible, by pitching over as hard as possible at the start, while still actually making orbit. Turn off the Max Q limiter (it might help some for 1.25m rockets, but it is fairly useless for anything larger).

Powered Explicit Guidance

More detailed information on the Surveyor-era PEG Ascent Guidance in MechJeb can be found here.

Target Options

Further options are available when a target is selected and the current ship is landed

TODO